Cap is probably an impractical idea, but all elements of Gov't should be looking to implement a reliable system of obtaining transparency regards earnings (I don't necessarily mean full public knowledge) so that those above a certain threshold are made to pay every penny of tax that they owe to the state - like the majority of us 'ordinary' people do.
Those who already earn a very comfortable amount, but are found to be attempting to defraud the country of more on the sly should be marched off to jail like the crooks that they are.

Utopia I know.